Field of the Invention
The present invention relates to ophthalmic lenses, and more particularly, to contact lenses designed to slow, retard, or prevent myopia progression. The ophthalmic lenses of the present invention comprise power profiles having positive spherical aberration for both small and large entrance pupil sizes, for example pupil sizes having a diameter of 3.0 mm to 7.0 mm, which are suitable for the use in controlling or reducing the progression of myopia.
Discussion of the Related Art
Common conditions which lead to reduced visual acuity include myopia and hyperopia, for which corrective lenses in the form of spectacles, or rigid or soft contact lenses, are prescribed. The conditions are generally described as the imbalance between the length of the eye and the focus of the optical elements of the eye. Myopic eyes focus in front of the retinal plane and hyperopic eyes focus behind the retinal plane. Myopia typically develops because the axial length of the eye grows to be longer than the focal length of the optical components of the eye, that is, the eye grows too long. Hyperopia typically develops because the axial length of the eye is too short compared with the focal length of the optical components of the eye, that is, the eye does not grow long enough.
Myopia has a high prevalence rate in many regions of the world. Of greatest concern with this condition is its possible progression to high myopia, for example greater than five (5) or six (6) diopters, which dramatically affects one's ability to function without optical aids. High myopia is also associated with an increased risk of retinal disease, cataracts, and glaucoma.
Corrective lenses are used to alter the gross focus of the eye to render a clearer image at the retinal plane, by shifting the focus from in front of the plane to correct myopia, or from behind the plane to correct hyperopia, respectively. However, the corrective approach to the conditions does not address the cause of the condition, but is merely prosthetic or intended to address symptoms.
Most eyes do not have simple myopia or hyperopia, but have myopic astigmatism or hyperopic astigmatism. Astigmatic errors of focus cause the image of a point source of light to form as two mutually perpendicular lines at different focal distances. In the following discussion, the terms myopia and hyperopia are used to include simple myopia or myopic astigmatism and hyperopia and hyperopic astigmatism respectively.
Emmetropia describes the state of clear vision where an object at infinity is in relatively sharp focus with the crystalline lens relaxed. In normal or emmetropic adult eyes, light from both distant and close objects and passing though the central or paraxial region of the aperture or pupil is focused by the crystalline lens inside the eye close to the retinal plane where the inverted image is sensed. It is observed, however, that most normal eyes exhibit a positive longitudinal spherical aberration, generally in the region of about +0.50 Diopters (D) for a 5.0 mm aperture, meaning that rays passing through the aperture or pupil at its periphery are focused +0.50 D in front of the retinal plane when the eye is focused to infinity. As used herein the measure D is the dioptric power, defined as the reciprocal of the focal distance of a lens or optical system, in meters.
The spherical aberration of the normal eye is not constant. For example, accommodation (the change in optical power of the eye derived primarily though changes to the crystalline lens) causes the spherical aberration to change from positive to negative.
U.S. Pat. No. 6,045,578 discloses that the addition of positive spherical aberration on the contact lens will reduce or control the progression of myopia. The method includes changing the spherical aberration of an ocular system to alter the growth in eye length. In other words, emmetropisation may be regulated by spherical aberration. In this process, the cornea of a myopic eye is fitted with a lens having increasing dioptric power away from the lens center. Paraxial light rays entering the central portion of the lens are focused on the retina of the eye, producing a clear image of an object. Marginal light rays entering the peripheral portion of the cornea are focused in a plane between the cornea and the retina, and produce positive spherical aberration of the image on the latter. This positive spherical aberration produces a physiological effect on the eye which tends to inhibit growth of the eye, thus mitigating the tendency for the myopic eye to grow longer.
Currently, the addition of the positive spherical aberration is defined in a manner that applies to only one specific pupil size. Since pupils, for example pediatric pupils, change dramatically with light level, there is a need to design optics that carries more consistent positive spherical aberration across various pupil sizes, for example, ranging from about 3 mm to about 7 mm in diameter, especially in the context of myopia control.